The ferrite, Alnico and rare-earth magnets have been used for various purposes, e.g., motors. However, these magnets are mainly produced by the sintering method, and have various disadvantages. For example, they are generally fragile and difficult to be formed into thin or complex-shape products. In addition, they are low in dimensional precision, because of significant shrinkage of 15 to 20% during the sintering step, and need post-treatment, e.g., grinding, to improve their precision.
On the other hand, resin-bonded magnets have been recently developed, in order to solve these disadvantages and, at the same time, to develop new applications. They are generally produced by filling them with a magnetic powder using a thermoplastic resin, e.g., polyamide or polyphenylene sulfide resin, as the binder.
However, the resin-bonded magnet with a thermoplastic resin as the binder involves characteristic disadvantages; its magnetic characteristics, in particular, coercive force and rectangularity, invariably deteriorate, because it is exposed to high temperature of 200° C. or higher while being formed. It has been difficult to produce the resin-bonded magnet with less deterioration of the magnetic characteristics, e.g., coercive force, after it is formed into a shape.
Other types of magnets proposed so far include the one filled with a magnetic powder for which a thermosetting resin, e.g., epoxy or bis-maleimidetriazine resin, serves as the binder. However, it can be formed only into a simple shape by compression molding, because of a limited amount of the binder included.
Under these circumstances, small-size motors, acoustic devices, OA devices or the like have been recently required to be even smaller, which requires the resin-bonded magnets therefor to have even more improved magnetic characteristics and more complex shapes. However, the resin-bonded magnets produced by the conventional method are insufficient in magnetic characteristics and shape complexity for the above purposes. Therefore, it is strongly desired to improve the resin-bonded magnets in the early stage.
Japanese Patent Laid-open Publication No. 10-7918 discloses a composition with an N-oxyl incorporated in a specific resin. However, it cannot produce the effects realizable by the present invention of a mixed composition containing metals including a transition metal, e.g., the effect produced by the mixed composition containing the metal in a high proportion and that by the composition containing an organic peroxide. These effects of the present invention can be sufficiently exhibited in the mixed composition which contains metals including a transition metal having a catalytic function for specific reactions and also an organic peroxide in the presence the metal in a high proportion. For example, the effects similar to those of the present invention cannot be realized by a mixed composition containing metals including a transition metal and binder composition composed of an azo-based polymerization initiator working similarly as a curing agent, the above-described specific resin and N-oxyl compound.
Japanese Patent Laid-Open No. 2001-11328 discloses a composition comprising a thermosetting resin composition composed of a specific resin and metallic compound having a redox function, incorporated with a phenyl sulfonate and N-oxyl compound.
However, it also cannot produce the effects realizable by the present invention of a mixed composition containing metals including a transition metal, e.g., the effect produced by the mixed composition containing the metal at a high proportion and that by the composition containing an organic peroxide. Moreover, the composition of the above patent publication contains an organometallic compound mainly to improve thermosetting characteristics of the composition, unlike the one as the essential component for the present invention. Its content is also very low. Therefore, it is very difficult for the composition to realize the effects of the present invention.
It is an object of the present invention to provide a resin-bonded magnet that is very excellent not only in magnetic characteristics but also excellent in shape freedom, moldability and mechanical strength by controlling oxidation-caused deterioration of the magnetic characteristics, from which the magnet produced by the conventional method suffers while being formed at a high temperature, and making it possible to highly orient an anisotropic magnetic material, for which orientation is very important. These favorable characteristics are produced by solving the problems involved in the resin-bonded magnet produced by each of the conventional methods, i.e., injection molding using a thermoplastic resin and compression molding using a thermosetting resin, the former giving a magnet of complex shape although low in magnetic characteristics, whereas the latter giving only a magnet of simple shape although high in magnetic characteristics. It is another object of the present invention to provide a composition for resin-bonded magnets with excellent service lives, which is most important for the magnet containing a thermosetting resin as the binder.